The invention relates to voltage controlled oscillators, and more particularly, to an oscillator module incorporating a looped-stub resonator.
Modern electronic systems often require a signal to be generated in the frequency range of a few MHz to thousands of MHz. Frequencies are generated through the use of oscillating circuitry and some form of frequency stabilizing resonant circuitry or element. A provision to control the frequency through a voltage is also generally provided and essential if the oscillator is to be used in a phase locked loop system (PLL). A basic PLL uses a voltage controlled oscillator (VCO) in conjunction with additional circuitry to control both the phase and frequency of the VCO. Various parameters such as cost, size, power, and other specifications are evaluated in determining the optimal design of the PLL.
In a conventional PLL, the output frequency is divided and the phase of this divided signal is compared to the phase of a reference signal input. An error signal proportional to the phase difference between the reference signal input and the divided output signal is generated by a phase detector circuit. This error signal is filtered and then used to control the frequency of the output frequency. The output frequency is equal to the input frequency multiplied by the division number.
The frequency divider may be programmable such that the output frequency become definable by the specific frequency division ratio. For example, if the input frequency is 10 MHz, and the output frequency is 1000 MHz, then the division ratio would be 100. If the division ratio is then changed to 90, then the output frequency would change to 900 MHz for the same 10 MHz input frequency. Various parameters such as the time necessary to perform the frequency change, along with the signal quality of the output frequency, are used to determine the proper design.
The circuitry used to filter the error signal from the phase detector is a low pass filter. This filter allows slowly varying voltages to pass on to the VCO, while attenuating high frequency or rapidly changing voltages. The bandwidth of the low-pass filter can vary from a few Hz to several MHz. For example, if it is desirable to rapidly switch between two frequencies, the low pass filter bandwidth is considerably larger. However, if a very pure output signal is required, then the low pass bandwidth can be narrower, with an attendant increase in switching time.
The performance of communication and instrumentation systems depends to a large degree on proper design and performance of phase locked loops. More specifically, the jitter and phase noise of the output frequency can affect many system specifications. Phase noise is a well-known impurity in frequency multiplication and synthesis. It is a measure of performance of the purity and stability of a signal. Phase noise is measured in the frequency domain and is expressed as the ratio of phase noise power to the signal power level in a 1 Hz bandwidth. For example, the phase noise of a 1000 MHz signal when measured at 100 kHz offset can be xe2x88x92160 dBc. Phase noise manifests in a number of ways in electronics systems. For example, phase noise in a PLL can mask the target signal in a radar system.
Jitter is closely related to phase noise and is a time domain parameter which describes the stability of a signal when measured over short periods of time. More specifically it is a parameter which describes the variation in the period of the signal over a defined measurement bandwidth. For example, the jitter of a 1000 MHz signal can be 1 ps over the bandwidth of 12 kHz to 20 MHz. Jitter can also be defined as a percentage of the total period of the signal. For the case of a 1000 MHz signal, the period will be reciprocal to the frequency, or 1 ns. Thus, 1 ps of jitter would be equivalent to 0.001 unit interval of one period. Jitter is an important parameter in communication systems and can induce error in the transmitted or received data.
A key attribute in the performance of a PLL is the phase noise of the VCO. At offset frequencies much less than the bandwidth of the low pass filter, the phase noise of the VCO will be related to the phase noise of the reference input with an additional contribution of 20 log (division ratio). For example, with a 10 MHz reference input and a 1000 MHz output, the phase noise at frequencies much less than the low pass filter bandwidth will be obtained from the input phase noise with an additional contribution of 60 dB. At frequencies much greater than the low pass filter bandwidth, the phase noise output signal will be directly related to the phase noise of the VCO. Therefore, the performance of the input reference signal, the VCO, and the low pass filter bandwidth all impact PLL performance.
The frequency of a VCO is primarily determined by the frequency of resonant elements. These elements must have some type of energy storage at a specific frequency. Common resonant elements are lumped element inductor-capacitor circuits and distributed resonant circuits. Phase noise of the VCO is determined to a large degree by the bandwidth of resonant elements in the VCO. The quality factor (Q) of the resonant circuit is determined by the amount of stored energy divided by the lost energy per cycle of resonance. An equivalent definition of Q is the ratio of the center frequency to the bandwidth of the resonant circuit. For example, a 1000 MHz VCO may have resonant circuit with a Q of 100.
In an oscillator, Q defines the offset frequency where phase noise begins to dramatically increase. Depending on circuit characteristics, the phase noise may increase by either 20 or 30 dB per decade at offset frequencies less than one half the center frequency divided by the Q. For the case of a 1000 MHz VCO with a Q of 100, the phase noise will begin to appreciably increase at frequencies less than 5 MHz.
In the case where inductors are integrated onto an integrated circuit (IC), substantial changes in frequency require a redesign of the IC. IC design and manufacture typically involve photolithographic techniques with circuit features determined by an optical mask. Redesign of an IC thus requires that at least one new photolithographic mask be created. Thus, one of the fundamental difficulties encountered in the design of PLLs and frequency synthesizers is obtaining adequate Q in the resonant circuitry of the VCO. Another difficulty is accomplishing the design associated with each new required frequency without the need to generate new photolithographic masks.
Distributed element resonant devices may also be used to stabilize the frequency of a VCO. The most common type is referred to as a stub, and is a straight line conductor surrounded by some type of insulating media and ground surface. The stub is a fraction of a wavelength and typically xc2xc or xc2xd of a wavelength. The inductance of the conductor and capacitance to the ground surface or plane serve as energy storage elements. The Q of distributed element resonant devices is often higher than lumped element inductor-capacitor circuits.
Common distributed element resonators are coaxial, microstrip stubs, stripline stubs, ring resonators and disk resonators. While having sufficiently high Q, these devices are physically too large for many applications and are generally incompatible with chip scale types of packaging. Stub devices have become quite popular due to their simplicity of design and low cost of manufacture. However, stub type must have a length which is a fraction of a wavelength and can become excessively long. At frequencies of 2 GHz, this length may be 1 inch or even longer, depending on the material. In short, conventional tuning techniques suffer from performance limitations, and/or have resonators that are physically too large for a given application.
What is needed, therefore, is a PLL module capable of meeting performance requirements while maintaining miniature dimensions. Further, the module should be capable of meeting various frequency requirements with only minor changes, rather than requiring a new mask.
One embodiment of the present invention provides a resonator device configured with an input port at one end and a termination at its other end, and for providing a frequency selective element for an oscillator. The device includes a substrate, and a fractional wavelength transmission line on a surface of the substrate. The transmission line is formed into one or more loops, thereby providing a looped-stub resonator structure. Each edge or side of the one or more loops provides a portion of the fractional wavelength (e.g., xc2xc or xc2xd wavelength).
The termination can be, for example, a capacitor, a short circuit, or an open circuit. In one particular embodiment, the device is a structure having a number of layers, and the transmission line is located in an inner layer of the structure. In one such an embodiment, the inner layer is substantially surrounded by dielectric insulating material layers. Here, electrically conducting material layers connected to ground may surround the dielectric insulating material layers.
The device can be incorporated, for example, into a voltage controlled oscillator of a phase locked loop circuit. Other circuits may also benefit, such as a frequency multiplication module or other frequency tunable applications. Note that the looped-stub resonator can be a metal pattern formed on the substrate, and changes in oscillation frequency can be accomplished by physically changing the metal pattern. In one such particular embodiment, the looped-stub resonator is formed on the substrate as a metal pattern that includes a capacitive termination, and changes in oscillation frequency are accomplished by physically changing the capacitive termination.
Another embodiment of the present invention provides a phase locked loop module. The module includes a voltage controlled oscillator circuit, and a fractional wavelength looped-stub resonator that is operatively coupled to the voltage controlled oscillator circuit. The looped-stub resonator has one or more loops, with each edge or side of the one or more loops providing a portion of the fractional wavelength. The looped-stub resonator provides a frequency selective element for the voltage controlled oscillator circuit.
In one such embodiment, the looped-stub resonator has a Q of 100 or greater. Note that the voltage controlled oscillator circuit and the looped-stub resonator can be located on a common substrate, or on different substrates (e.g., in a layered structure). In another particular embodiment, the module includes a number of layers and the looped-stub resonator is located on a layer that is above a dielectric insulation layer. Here, the dielectric insulation layer can be located above an electrically conducting material layer that is connected to ground.
The looped-stub resonator can be a metal pattern on a substrate, and changes in oscillation frequency can be accomplished by physically changing the metal pattern. In one such embodiment, the looped-stub resonator is on a substrate as a metal pattern that includes a capacitive termination, and changes in oscillation frequency are accomplished by physically changing the capacitive termination. In another particular embodiment, the looped-stub resonator has a resonant frequency higher than an output frequency of the module. In such a case, one or more frequency dividers can be used to reduce the resonant frequency to the output frequency.
Another embodiment of the present invention provides a phase locked loop module. The module includes a first layer having a voltage controlled oscillator circuit, and a second layer of dielectric insulating material covered with a conducting metal that is connected to a ground plane. A third layer having a fractional wavelength looped-stub resonator that is operatively coupled to the voltage controlled oscillator circuit. The looped-stub resonator has one or more loops, with each edge or side of the one or more loops providing a portion of the fractional wavelength. The resonator provides a frequency selective element for the voltage controlled oscillator circuit. A fourth layer of dielectric insulating material covered with a conducting metal that is connected to the ground plane, wherein the third layer is surrounded by the dielectric insulating material of the second and fourth layers.
In one such embodiment, the module further includes an additional layer of dielectric insulating material on the conducting metal of the second layer to prevent unintended short-circuiting between the first layer and the second layer. In another such embodiment, the looped-stub resonator has a resonant frequency that is higher than the output frequency of the module. One or more frequency dividers can be used to reduce the resonant frequency to the output frequency.
The features and advantages described herein are not all-inclusive and, in particular, many additional features and advantages will be apparent to one of ordinary skill in the art in view of the drawings, specification, and claims. Moreover, it should be noted that the language used in the specification has been principally selected for readability and instructional purposes, and not to limit the scope of the inventive subject matter.